Ultrasonic technique to measure helium abundance on the giant planets: An experimental validation study

Helium abundance on Saturn remains a long-standing open question, limiting understanding of theories of planetary formation, and also holds relevance to atmospheric dynamics. In this work, an ultrasonic sensing technique for measuring helium abundance in mixtures of helium and hydrogen gas is investigated and experimentally demonstrated. Ultrasound-based techniques using the speed of sound and absorption spectra of hydrogen-helium gas mixtures are a candidate for measuring helium abundance, and possibly the hydrogen ortho/para fraction, in the atmospheres of the giant planets, particularly Saturn and Uranus. In the present work, the technique is demonstrated experimentally to measure helium abundance in H2:He mixtures at 0.5-7 bars and 292 K over a frequency range of 100 kHz-1 MHz. The measured sound speed and absorption are compared with theoretical results. The uncertainty in the H2:He ratio in the current test apparatus due to the limitation of the chamber pressure gauges is as high as ±16% under some conditions. The ultrasonic technique outperforms the pressure gauge uncertainties: A maximum error of -11.9% is observed, which is likely driven by the gas composition accuracy, not the accuracy of the technique. This work is the beginning of the process of demonstrating a prototype instrument for a planetary descent probe targeted at Saturn or Uranus.